Is IEEE 802.11 TSF Scalable?

1. Is IEEE 802.11 TSF Scalable?

2. IEEE 802.11: how large can it be? • Bandwidth: • Up to 54 Mbps • Good for a few hundred nodes • Timing Synchronization Function • Not scalable • How to fix it?

3. 802.11’s Time Sync Function (I) • Time divided into beacon intervals, each containing a beacon generation window. • Each station: • waits for a random number of slots; • transmits a beacon (if no one else has done so). • Beacon: several slots in length. beacon interval window

4. 802.11’s Time Sync Function (II) • Beacon contains a timestamp. • On receiving a beacon, STA adopts beacon’s timing if T(beacon) > T(STA). • Clocks move only forward. 12:01 12:02 12:01 12:00 12:01 faster slower adopts not adopts

5. Problems with 802.11’s TSF • Faster clocks synchronize slower clocks. • Equal opportunity for nodes to generate beacons. 1:16 1:17 1:18 1:19 1:21 1:23 1:21 1:22 1:23 1:25 1:28 1:31 1:18 1:18 1:18 1:19 1:21 1:23 1:23 1:23 1:23 1:25 1:28 1:31 1:10 1:11 1:12 1:13 1:14 1:15 1:13 1:13 1:13 1:13 1:14 1:15 +3 +4 +5 +6 +7 +8 +3 +4 +5 +6 +7 +8

6. The Out-of-Sync Problem When number of stations increases Fastest station sends beacons less frequently Stations out of synchronization

7. Two Types of Out-of-Sync • Fastest-station out-of-sync – fastest station is out of sync with all others. • k-global out-of-sync – k percent of links are out of sync. • Questions: How often? For how long?

8. Fastest-station out-of-sync (1) • Clock1 and Clock2: two fastest clocks • d = their difference in accuracy • T = length of beacon interval (0.1 sec.) • Clock drift: d*T per beacon interval. • In /(d*T) intervals, fastest-station will be out of sync with all others. T

9. Fastest-station out-of-sync (2) • n = number of stations. w = size of beacon window. • P’(n,w) = prob(fastest station wins beacon contention)

10. Prob(Fastest station sends a beacon)

11. Fastest-station out-of-sync (3) • H = # beacon intervals with F.S. out-of-sync. • L = # beacon intervals between async periods. • E(R) = E(H)/[E(H)+E(L)] = percent of time in which the fastest station is out of sync with all others. H L

12. How often does fastest-node get out of sync with others?

13. Percentage of time fastest station out of sync with all others 802.11a 54 Mbps ∆ = 224 s d = 0.003%

14. How often does 25%-async occur?

15. Percentage of time with 25 percent of links out-of-sync 802.11a 54 Mbps ∆ = 224 s d = 0.01%

16. How to fix it? • Desired properties: simple, efficient, and compatible with current 802.11 TSF. • Causes of out-of-sync • Unidirectional clocks • Equal beacon opportunity • Single beacon per interval • Beacon contention (collision) 1 n Prob <

17. Improve fastest station’s chance • Let the fastest station contend for beacon generation more frequently than others.

18. Adaptive Clock Sync Protocol • Station x participates in beacon contention once every C(x) intervals. • Initially, C(x) =1. Always, 1 < C(x) < Cmax. • Dynamically adjust C(x): x x C(x) +1 faster C(x) -1 slower

19. Once the protocol converges Fastest station, C(x) =1 Other stations, C(x) = Cmax (Cmax= ?)

20. What if the fastest node leaves the IBSS? • The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.

21. What if a new fastest node enters the IBSS? • The previously fastest now no longer the fastest. Its C(x) will increase to Cmax.

22. Compatible with current TSF • Suppose some nodes do not implement the new protocol.

23. Performance • 802.11 Performance of TSF • ATSP ATSP.pdf • TATSP Performance of Modified TSF

24. Performance of TSF

25. Performance of ATSP

26. Performance of Modified TSF

27. Modified TSF • Divide stations into three groups: • Group 1: C(x) = Cmax1 = 1 • Group 2: C(x) = Cmax2 = a small number • Group 3: C(x) = Cmax3 = a large number

28. Summary • Showed: the IEEE 802.11 Timing Sync Function (TSF) is not scalable. • Proposed: a simple remedy compatible with the current TFS.

29. What’s Next? • IBSS: single-hop • MANET: multihop transmission range